This invention relates to the treatment of a subterranean petroleum formation to enhance oil recovery and more particularly to the treatment of a subterranean petroleum formation, containing two or more zones which differ from one another in permeability, with zirconium gel compositions to enhance oil recovery.
It is well known that only a small fraction of the petroleum present in a subterranean petroleum formation is recoverable by primary production i.e. by allowing the oil to flow to the surface of the earth as a consequence of naturally occurring energy forces. In many instances, secondary waterflooding of many reservoirs recovers only a small portion of the remaining oil because of the presence within the reservoir of a number of strata of widely varying permeability.
In general, secondary recovery processes comprise injecting fluids into the formation via one or more injection wells to displace petroleum through the formation and finally to the surface of the earth. Fluids used in such processes include liquids, such as water and various hydrocarbons and gases, such as carbon dioxide and the like. In all fluid drive processes a recognized problem is the predilection of the drive fluid to pass through the more permeable zones of the formation. The more permeable or conductive zones, after the oil has been largely displaced therefrom, function as thief zones which permit the drive fluid to channel directly from injection to production wells.
To achieve higher sweep efficiency, the permeability of the loose (more permeable) zones must be reduced so that the flooding fluid may invade the previously underswept less permeable (tight) zones to displace the oil. Various polymer gels have been used for this purpose. One of the most widely employed gel types is the Cr(III) crosslinked polymer containing carboxylate groups, such as Cr-partially hydrolyzed polyacrylamide gels and Cr-xanthan biopolymer gels.
In a recent patent, namely U.S. Pat. No. 4,460,751, certain zirconium (IV) type gels are disclosed as being useful to achieve higher sweep efficiency in secondary oil recovery processes. Zirconium is reported to be nontoxic and in addition, Zr(IV) is known to interact strongly with carboxylate groups to form complexes which are more stable than those formed by using Cr(III). Thus, gels capable of withstanding higher temperatures than chromium gels may be prepared from zirconium crosslinking.
Notwithstanding their apparent advantages, zirconium gels are difficult to prepare. For example, reacting Zr(IV) reagents with carboxylate containing polymers can result in lumpy and heterogeneous gels of undesired morphology. Within such gels, regions of high crosslinking density are formed which may eventually lead to the destruction of the gel structure.
In said U.S. Pat. No. 4,460,751, strong complexing agents, such as alpha-hydroxy-carboxylic acids and alkanol amines are added to Zr(IV) reagents to retard the gelation reaction by masking portions of the reactive sites of the zirconium reagents. While this technique reduces the reactivity of the zirconium reagents and allows the zirconium reagents to be dispersed evenly to form stable gels without precipitation, it is found that the effectiveness of the zirconium is reduced and a higher initial zirconium concentration is required to offset the reactivity lost by the added retardants.
Thus, it is apparent that a permeability reducing crosslinked gel composition, wherein the advantages of the use of zirconium are realized, and wherein there is rapid gelation of the polymer without the need for high concentrations of zirconium, is desirable.